from the fibres [31]. With high polymer addition levels (as much as 40% of the dry
fibre), the tensile strength was increased up to three times, but other problems such
as stickiness, lowered dewatering rates, flocculation and translucence arose [31].
Another study showed that in-situ formed complexes (from PDADMAC and CMC)
yielded a higher tensile strength of sheets formed from glass fibres than pre-formed
PECs of the same polyelectrolytes [32], emphasising the great influence of the
chemical addition strategy used. The glass fibres were used as a simplified model of
cellulosic fibres in order to have a more defined system with, e.g. no fines, no
swelling and no intrafibre porosity. Also, sheets formed from glass fibres without
any polyelectrolytes have almost no cohesive strength, so that any increase in
tensile strength with added polymer can be easily ascribed to the polymer addition.
The sequential addition of two oppositely charged polyelectrolytes is a commonly used industrial method for increasing the retention of filler and fine material,
using so-called dual (component) retention aids [33]. The polyanions can also be
replaced by anionically charged micro- or nanoparticles, as in the microparticulate
retention aids [34]. Addition of the polycation first, followed by the polyanion, is an
analogue to building the first two layers of a PEM structure [5], which was first
introduced in the paper field by Wa ˚gberg et al. [6], and later studied extensively by
several research teams [35–37].
3.2 In Situ PEC Formation with Wood Components
A special case of in situ PEC formation is to utilize a cationic polyelectrolyte together
with the anionic material already present in the pulp suspension (i.e., in different
wood components). Besides pulp fibres, fines and filler material, the paper furnish can
also contain a considerable amount of molecules originating from the wood material.
Native wood fibres are composed of cellulose, hemicellulose and lignin as major
constituents, roughly one third of each, and, as previously mentioned, the main
purpose of the pulping process is to liberate the wood fibres from each other to
form a suspension of free fibres. Although the two principal pulping techniques
(chemical and mechanical) are very different from each other, the wood compounds
released to the process waters are basically the same, i.e., originating from lignin or
hemicellulose, although the types and relative concentrations may differ. Other
processes, such as bleaching and beating, may result in a further dissolution of
chemical compounds from the fibre material into the aqueous phase. These dissolved
and colloidal substances (DCS) are often polymeric substances with negative charges
and, due to their ability to intervene with the papermaking process, they are often
condescendingly referred to by papermakers as anionic trash. When cationic
polymers (e.g., retention additives) are added to the fibre suspension, PECs can
form spontaneously between the added polycations and these anionic wood
components.
With today’s striving towards more environmentally sound production processes
and low water usage (especially in countries with water shortage), increased system
10
C. Ankerfors and L. Wa ˚gberg
fibre), the tensile strength was increased up to three times, but other problems such
as stickiness, lowered dewatering rates, flocculation and translucence arose [31].
Another study showed that in-situ formed complexes (from PDADMAC and CMC)
yielded a higher tensile strength of sheets formed from glass fibres than pre-formed
PECs of the same polyelectrolytes [32], emphasising the great influence of the
chemical addition strategy used. The glass fibres were used as a simplified model of
cellulosic fibres in order to have a more defined system with, e.g. no fines, no
swelling and no intrafibre porosity. Also, sheets formed from glass fibres without
any polyelectrolytes have almost no cohesive strength, so that any increase in
tensile strength with added polymer can be easily ascribed to the polymer addition.
The sequential addition of two oppositely charged polyelectrolytes is a commonly used industrial method for increasing the retention of filler and fine material,
using so-called dual (component) retention aids [33]. The polyanions can also be
replaced by anionically charged micro- or nanoparticles, as in the microparticulate
retention aids [34]. Addition of the polycation first, followed by the polyanion, is an
analogue to building the first two layers of a PEM structure [5], which was first
introduced in the paper field by Wa ˚gberg et al. [6], and later studied extensively by
several research teams [35–37].
3.2 In Situ PEC Formation with Wood Components
A special case of in situ PEC formation is to utilize a cationic polyelectrolyte together
with the anionic material already present in the pulp suspension (i.e., in different
wood components). Besides pulp fibres, fines and filler material, the paper furnish can
also contain a considerable amount of molecules originating from the wood material.
Native wood fibres are composed of cellulose, hemicellulose and lignin as major
constituents, roughly one third of each, and, as previously mentioned, the main
purpose of the pulping process is to liberate the wood fibres from each other to
form a suspension of free fibres. Although the two principal pulping techniques
(chemical and mechanical) are very different from each other, the wood compounds
released to the process waters are basically the same, i.e., originating from lignin or
hemicellulose, although the types and relative concentrations may differ. Other
processes, such as bleaching and beating, may result in a further dissolution of
chemical compounds from the fibre material into the aqueous phase. These dissolved
and colloidal substances (DCS) are often polymeric substances with negative charges
and, due to their ability to intervene with the papermaking process, they are often
condescendingly referred to by papermakers as anionic trash. When cationic
polymers (e.g., retention additives) are added to the fibre suspension, PECs can
form spontaneously between the added polycations and these anionic wood
components.
With today’s striving towards more environmentally sound production processes
and low water usage (especially in countries with water shortage), increased system
10
C. Ankerfors and L. Wa ˚gberg